Originally Posted by PHLDividends

You can book on united.com, and then call United and they will add your US number to the reservation - I did that just last week. You can try entering it online, but I think it's kind of sketchy as to whether it winds up there or not if you don't call.

I don't know the answer to your second question - you can certainly book US flights with United miles if they are code-shared as United flights, which may be your best bet.

Good luck!